Along with a remarkable breakthrough in the field of wireless communications, a variety of developments have been in progress in order to accelerate information transmission. With the wireless communications, frequency bands of approximately 2 GHz are in wide use in order to cope with the introduction of PHS systems, third generation cellular phones, wireless LAN, and so on. Further, the number of users and wireless terminals are extensively increased. The higher the information transmission speeds, the higher the carrier frequencies. Wireless LAN systems operating on a 5 GHz frequency band are now in business use.
There are strong demands on miniaturization of communication devices operating on high frequency bands. Especially, with a personal computer (PC), a communication device is realized by a PC card which should be as thin as approximately several millimeters.
Generally, a wireless communication device in the shape of the PC card includes an RF front end which processes radio frequencies, and a base band (BB) unit which processes digital signals. The base band unit is preferably an LSI chip made of a silicon (Si) substrate, and can be thinned to 1 mm or less.
The RF front end amplifies and coverts high frequencies as analog signals, and includes a number of passive components such as oscillators and filters. It is technically difficult to constitute the RF front end only by an LSI chip because the RF front end has a very complicated structure. The filters are either dielectrics or LC filters. The filters can filter high frequency signals using passband characteristics of a cavity resonator or an LC circuit, and is essentially difficult to be downsized and to be thinned to several millimeters or less. In other words, communication devices operating on high frequency bands have limitations on their miniaturization.
In order to overcome the foregoing problem, Japanese Patent Laid-Open Publication No. 2000-069,594 proposes a film bulk acoustic wave resonator (FBAR) which has attracted attentions, for example. In the FBAR, a thin piezoelectric film made of aluminum nitride (AlN) or zinc oxide is sandwiched between lower and upper electrodes. The thin piezoelectric film is placed over a cavity in a substrate. The resonator lets frequencies resonate along the thickness of not only the lower and upper electrodes which are in contact with an air layer but also the piezoelectric film. The foregoing thickness of 0.5 μm to several μm which can be accomplished by a film making process is suitable to frequencies of several GHz. Therefore, resonators compatible with high frequencies in GHz bands can be easily manufactured.
For instance, two thin film piezoelectric resonators are connected in series, and one thin film piezoelectric resonator is connected in parallel with the two thin film piezoelectric resonators, thereby obtaining a ladder-shaped filter. With a passband filter, the central frequency of the series-connected this film piezoelectric resonators and that of the parallel-connected thin film piezoelectric resonator are slightly different. Therefore, the resonance frequency of the parallel-connected thin film piezoelectric resonator is adjusted to be equal to that of the series-connected thin film piezoelectric resonators, for instance.
The foregoing thin film piezoelectric resonator can be produced using the film making process which is used to form a thin film on a substrate, and can be miniaturized. Especially, a general purpose filter can be easily made as thin as 1 mm or less, which is usually very difficult. Further, the substrate may be made of Si, which enables the thin film piezoelectric resonator to be produced by a semiconductor manufacturing process. Still further, the thin film piezoelectric resonator is reliably compatible with a transistor, IC, LSI and so on, and can have them mounted thereon.
However, there are the following new problems when a high frequency module is made using the thin film piezoelectric resonator on which a transistor, IC, LSI and so on are mounted.
The thin film piezoelectric resonator operates on bulk standing waves which are generated in a direction extending along the thickness of the piezoelectric film and produces resonance. However, lateral mode standing waves are generated at an edge of an electrode and an edge of a piezoelectric film. Such lateral mode standing waves have specific values. As a result, a lateral mode standing wave is generated. A wavelength of the lateral mode standing wave differs from that of the bulk wave. When combined with the bulk wave, the lateral mode standing wave causes a variety of parasitic vibration modes (spurious vibrations). If spurious vibrations are caused, ripples are generated, which fluctuate high frequency signal characteristics (Smith chart). This phenomenon extensively deteriorates resonance performance of the thin film piezoelectric resonator, or makes the resonance performance variable.
In order to overcome the foregoing problem, it has been proposed to suppress the lateral mode standing wave by making a contour of an upper electrode of a thin film piezoelectric resonator in the shape of an irregular polygon, as shown in FIG. 14 of the accompanying drawings. However, since such an upper electrode 104 becomes large, it is impossible to miniaturize a filter including thin film piezoelectric resonators.
Referring to FIG. 15 and FIG. 16, a thin film piezoelectric resonator 100 includes an upper electrode 104, a substrate 101 having a cavity 101H, a lower electrode 102 extending over the cavity 101H, and a thin piezoelectric film 103 on the lower electrode 102. The upper electrode 104 is present over the piezoelectric film 103, and has a damping layer 105 at its one end. The damping layer 105 damps the lateral mode standing wave. However, a new process for making the damping layer 105 should be added to a process for making the thin film piezoelectric resonator 100. This not only increases the number of manufacturing processes but also reduces an yield of the thin film piezoelectric resonator 100. Further, the damping layer 105 should be aligned with the edge of the upper electrode 104. However, a sufficient processing margin cannot be secured.